Typical wireless power transmission systems adopting a magnetic coupling method are known. In the magnetic coupling method, power is transmitted from a primary coil in a power transmission apparatus to a secondary coil in a power receiving apparatus through a magnetic field. However, since the magnitude of the magnetic flux passing through each coil has large effect on the electromotive force when the power is transmitted by the magnetic coupling method, high-accuracy relative position relationship is required between the primary coil and the secondary coil. In addition, since the coils are used, it is difficult to reduce the sizes of the apparatuses.
Wireless power transmission systems adopting an electrical coupling method are also known. Such a wireless power transmission system is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2009-296857. In this wireless power transmission system, power is transmitted from a coupling electrode in a power transmission apparatus to a coupling electrode in a power receiving apparatus through an electric field. In the electrical coupling method, the relative position accuracy required for the coupling electrodes is relatively low and the coupling electrodes can be reduced in size and thickness.
FIG. 1 is a block diagram showing the configuration of a power transmission system 100 disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857. Referring to FIG. 1, the power transmission system 100 includes a power feeding apparatus 152 and a power receiving apparatus 154. The power feeding apparatus 152 includes a resonance unit 62 and power feeding electrodes 64 and 66. The power receiving apparatus 154 includes power receiving electrodes 80 and 82, a resonance unit 184, a rectification unit 86, a circuit load 88, a power measuring unit 120, and an impedance control unit 130. The power measuring unit 120 detects voltages at both ends of the circuit load 88 to measure the power value that is currently being supplied to the circuit load 88 and supplies the measured power value to the impedance control unit 130. The impedance control unit 130 controls the voltages at both ends of a variable capacitive element Cv1 using, for example, a variable capacitance element or the inductance of a variable inductive element Lv1 on the basis of the power value supplied from the power measuring unit 120 to maximize the power value that is being supplied.
Japanese Unexamined Patent Application Publication No. 2008-236968 discloses a wireless power transmission system composing a charging apparatus, in which recharge after full charge of a secondary cell is considered.
FIG. 2 is a block diagram showing the configuration of the power transmission system disclosed in Japanese Unexamined Patent Application Publication No. 2008-236968. Referring to FIG. 2, a power transmission apparatus 1 includes an oscillation (osc.) circuit 11, a driving clock generating circuit 12, a driver control circuit 13, driver circuits 14a and 14b, capacitors 15a and 15b, a primary coil 16, a current detecting circuit 17, and a control circuit 18.
The primary coil 16 is electromagnetically coupled to a secondary coil 21 in a power receiving apparatus 2 to transmit power from the primary coil 16 to the secondary coil 21 by using electromagnetic induction. The current detecting circuit 17 detects a current flowing through the primary coil 16. The detected current is supplied to the control circuit 18. The control circuit 18 performs certain power feed control on the basis of the current detected by the current detecting circuit 17.
The power receiving apparatus 2 receives power transmitted from the power transmission apparatus 1 and uses the power to charge a secondary (sec.) cell 26. The power receiving apparatus 2 includes the secondary coil 21, a rectifier circuit 22, a smoothing capacitor 23, a regulator 24, a monitor circuit 25, and the secondary cell 26.
The secondary coil 21 is electromagnetically coupled to the primary coil 16 in the power transmission apparatus 1 to induce a voltage. The primary coil 16 and the secondary coil 21 are each composed of a planar coil in which a winding is spirally wound on the same plane. The planes are opposed to each other and are close to each other to cause the electromagnetic induction. The rectifier circuit 22 rectifies the voltage induced in the secondary coil 21. The smoothing capacitor 23 smoothes the voltage output from the rectifier circuit 22. The smoothed voltage is supplied to the regulator 24.
The regulator 24 generates a desired stable voltage on the basis of the smoothed voltage. The generated voltage is supplied to the monitor circuit 25 and the secondary cell 26. A load 27 is connected to the secondary cell 26. The monitor circuit 25 operates in response to the voltage output from the regulator 24 to monitor the voltage of the secondary cell 26 and the current therethrough.